As described in U.S. Pat. No. 3,420,139 a standard breech mechanism of a cannon has a barrel formed with a backwardly open shell chamber, a breech block displaceable into a position rearwardly closing the chamber, and a seal ring between the block and the barrel and having an inner face exposed in the chamber and an outer face lying against the block and barrel. This seal ring is typically of L-section, having a cylindrical forward flange that fits against the inner surface of the rear end of the barrel and a rear flange that lies against the front face of the breech block.
Obviously this part is subject to considerable heavy-duty stress. It is exposed to extremely high pressures and temperatures with every shot, and can become extremely hot when a volley is being fired. In addition during each reloading operation the breech block slides off its outer face of the rear flange, a spent casing is pulled out through it, a new shell is loaded into the chamber through the ring, and the block slides back into place against the seal.
This seal ring wears out so rapidly that it is made extremely easy to replace in the barrel chamber and the gun crew is normally capable of putting in a new seal in a very short time. A supply of such seals is kept on hand in the field for this purpose.
Even though the above-mentioned patent provides a mechanism that locks out the breech when a sensor detects that there is no ring in the mechanism, as for instance happens occasionally when the casing-ejector hook catches on the ring and pulles it out with the shell, the crew is still exposed to the possibility of there being some leakage through the seal when it merely is damaged slightly or punctured somewhere. In this situation firing gases shoot out between the block and barrel, exposing the crew to these nasty vapors while causing an inevitable loss in muzzle velocity and elevation. This is particularly a problem when operating at night, under actual combat conditions, or when for other reasons the ring cannot be inspected visually each time the gun is reloaded.